Alphaviruses, such as Eastern and Western equine encephalitis viruses, are important causes of mosquito-borne encephalitis in the Americas, and infected individuals have disease manifestations ranging from a mild febrile illness to fatal encephalitis. The outcome of viral encephalitis is determined both by the ability of neurons to resist infection and the virulence of the infecting virus. Neurovirulence is a complicated biologiC property of a virus and encompasses many aspects of in vivo replication: neuroinvasiveness, cellular tropism and cellular damage. We have shown that strains of sindbis virus (SV) differ widely in neurovirulence, but this is not associated with a change in the cellular targets within the nervous system. All strains of SV infect neurons, but differ in the degree of neuronal damage inflicted. Previous studies have shown that SV produces neuronal cell death by inducing apoptosis and have identified the E2 glycoprotein as a major determinant of virulence with probable contributions from the E1 glycoprotein. We have also shown that the more neurovirulent strains of SV are able to overcome the block to apoptosis provided by cellular proteins expressed by mature neurons. The study of SV neurovirulence offers the opportunity to define at a molecular level the contributions of specific amino acid changes and the mechanisms by which they alter virulence. Studies to date have indicated that each amino acid change has a distinct effect on virus replication. Based on previous studies and preliminary data the aims of the research are: (1) To determine the role of amino acid changes in the E2 glycoprotein in the interaction of SV with receptors on different cells. (2) To determine the role of amino acid changes in the E1 and E2 glycoproteins in virus entry into cells of different types and the potential role of cellular fusion proteins in the entry process. (3) To identify the role of structural proteins in tissue tropism and in establishing efficient replication complexes using chimeric SV-Ross River virus chimeras (4) To determine the role of amino acid changes in the E1 and E2 glycoproteins in induction of apoptosis in neurons. (5) To determine the role of amino acid changes in the E1 and E2 glycoproteins in regulation of Na+K+ATPase activity and virus maturation.